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Generators & Motors General Discussion Antique Generators, Light Plants and Old Electric Motors: Questions and answers about restoring and showing old power generation systems.

Generators & Motors General Discussion

Light Bulb Current Question


I have noticed that smaller automotive light bulbs are frequently rated in C.P. Is there a constant...

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Old 02-19-2008, 10:08 AM
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Default Light Bulb Current Question

I have noticed that smaller automotive light bulbs are frequently rated in C.P. Is there a constant formula to convert C.P. to amps (or mA)? I am wanting to make a wall decoration at the shop from a complete 1970 Thunderbird tail light panel assy. Including the back-up lamps there are 12 light bulbs involved. If I go 'simple' and put (12) 1156 bulbs in all the holes, how many amps will I draw? If I manage to build it up to the 'deluxe' version with active turn signals, there will be (6) 1156 bulbs & (6) 1157. I have a multi position rotary switch (from an 6 speed hand mixer) that I have coupled to a 10rpm timing motor to simulate the sequential turn signal function that this car had.
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Old 02-19-2008, 10:23 AM
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KeithW KeithW is offline
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Default Re: Light Bulb Current Question

A quick search turned up 2.1 amps. I also found 21 watts, a little less that 2 amps but pretty close. Don't know of a formula to convert cp to amps. Depends on the efficiency of the bulb design.

keithw
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Old 02-19-2008, 10:47 AM
Patrick McNallen Patrick McNallen is offline
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Default Re: Light Bulb Current Question

The switch will need to handle the higher amp draw of the cold bulbs. A power supply & transistorized switching device that could turn left or right & operate both both left & right together with a variable speed function would be neat. Bulb burnout might be a problem if it ran much. LEDs might replace bulbs for lower current draw & no burnout problem. I remember those cars on the road, and they sure got attention!
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Old 02-19-2008, 10:51 AM
Stephen Girouard Stephen Girouard is offline
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Default Re: Light Bulb Current Question

i'd hook my battery charger to it and see how high the needle jumps, i like decorations like that
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Old 02-19-2008, 12:23 PM
Merritt Merritt is offline
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Default Re: Light Bulb Current Question

i dont know how you are with electronics, but when i was taking digital logic a few years ago, we had a test question that was exactly this, it even said it was for a thunderbird.. lol.

anyhow, its easy enough to do.. if you'd like i can provide you a simple schematic you can solder together, or i can build it and sell it to you...

it can either be made with flip-flops, a 555 timer and some relays to switch the bulbs, or it can be made with a 4017 decade counter, a 555 timer and some relays.

let me know
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Old 02-19-2008, 12:40 PM
Don Smith Don Smith is offline
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Default Re: Light Bulb Current Question

I too would suggest using the new super bright LED's. They last a lot longer, have no starting current surge as do cold bulbs, and use a lot less power to provide the bright light needed. they can run on lower voltage or be wired in series to run on 12 vdc, and can be interfaced with simple logic to providing the switching needed to give the turn signal flashing. There are simple electronic kits made by Ramsey Electronics that can provide the switching of the LED's for under $20. They also have the LED's I believe. Don
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Old 02-19-2008, 03:01 PM
armandh armandh is offline
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Default Re: Light Bulb Current Question

john I have just the transformer /dc supply you need

transformer rated at 37.5 amps @ 12vac
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Old 02-19-2008, 03:10 PM
armandh armandh is offline
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Default Re: Light Bulb Current Question

or just put 10 identical bulbs in series
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Old 02-19-2008, 09:01 PM
Kimbra Dean Kimbra Dean is offline
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Default Re: Light Bulb Current Question

If you do use incandescents and plan to flash them a lot then you should keep each bulb warm by supplying power to it through a resistor even when it is supposed to be off. Size the resistors so the bulb filament just barely glows. This will greatly reduce the turn on current and increase lamp life.
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Old 02-20-2008, 10:19 AM
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Default Re: Light Bulb Current Question

Thanks guys!
Lots of good ideas.
The rotary switch I am planning to use should be able to handle the current. The mixer it came out of had a 3A universal (brushes) motor. I am mostly concerned that the "on" time for each contact may be very brief and might not let the bulb come up to full brightness. The suggestions for a transistorized flasher circuit are good, but I am hoping to keep cost at an absolute minimum. Hopefully I will have a picture to add here sometime soon.....
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